In “direct” printing machines, a marking material is applied directly to a final substrate to form the image on that substrate. Other types of printing machines utilize an “indirect” or an “offset” printing technique. In this process the marking material is first applied onto an intermediate transfer member, and is subsequently transferred to a final substrate.
In one type of indirect printing machine, a piezoelectric ink jet printhead is used to apply melted solid ink to the intermediate transfer material layer. The solid ink is disposed on a liquid layer in the form of a release agent, such as oil, that is capable of supporting the printed image for subsequent transfer. The intermediate image is transferred by contact between the transfer drum and the substrate, typically with the assistance of a pressure roller or drum. An exemplary indirect printing apparatus 10 is shown in FIG. 1. In this apparatus, a printhead 11 directs a marking material, such as molten ink droplets, onto a layer 12 of intermediate transfer material to form an image 26. This transfer material layer 12 is carried by an intermediate transfer member 14, which in the illustration is a rotating drum or roller. An optional heater 19 may be provided to ensure that the ink image 26 remains molten prior to contacting the substrate 28.
The substrate 28 is conveyed between the intermediate transfer member 14 and a transfer or pressure roller 22. Optional heaters 20 and 21 may be provided to pre-heat the substrate 28 to facilitate reception of the image. Likewise, an optional heater 24 may be provided to heat the transfer roller 22. As the substrate is conveyed between the rotating rollers 14 and 22, the image 26 is transferred onto the substrate as image 26′. Appropriate pressure is maintained between the two rollers so that the image 26′ is properly spread, flattened and adhered onto the substrate 28. An optional stripper 25 may be provided that assists in removing any ink remaining on the intermediate transfer member 14 prior to receiving a new ink image 26 from the printhead 11.
As shown in FIG. 1 the apparatus 10 further includes an applicator 15 that is used to apply the liquid release layer 12 onto the intermediate transfer member. The applicator 15 is mounted on a movable platform 17 that moves the applicator into contact with the intermediate roller 14 between operations of the printhead 11. A metering blade 13 is provided that meters the thickness of the liquid layer 12 as it is applied. The release layer or transfer material may be an oil, such as a fluorinated oil, mineral oil, silicone oil or certain functional oils suitable for maintaining good release properties of the image transfer member. Using the metering blade 13, the applicator 15 applies a uniform coating of the transfer material, often ranging from a thickness of 0.02 micrometer to 1.0 micrometer and above, depending upon the surface characteristics and topography of the transfer drum 14. For instance, in some transfer drums the surface onto which the transfer material is applied can have an average roughness of about 0.01 micrometers to 0.60 micrometers.
It has been found that a certain amount of surface roughness or texture on the transfer drum 14 is desirable. If the roller surface is too smooth it does not provide sufficient oil retention which allows for robust and efficient image transfer. The roughness also helps pin the image drops so that the drops cannot flow or shift as they solidify or as they are transferred from the drum 22 onto the substrate 28. On the other hand, a surface that is too rough is also undesirable. High drum surface roughness leads to low gloss levels on the final image. It can also lead to an increase in consumption of release agent material and abrasion of the other working components of the machine, such as the applicator 15, metering blade 13 and the stripper 25. Abrasion of the metering blade 13 can be particularly problematic because abrasion can compromise the ability of the blade to produce a sufficiently low and uniform release layer 12 across the entire width and circumference of the drum 14. Moreover, as the metering blade wears the thickness of the release layer 12 increases. This leads to increased oil consumption and also degradation of print quality, especially in duplex printing modes. Also, increased oil consumption can lead to increases in operational costs. On the other hand, a very low surface texture or a surface that is too smooth (i.e., low oil retention) can lead to stripper smudges, high gloss levels and/or image dropout on the printed image.